1. Field of the Invention
The present invention relates to methods for purifying tryptophan (Trp) which comprise removing impurities, such as proteins and colored substances, from Trp obtained from a Trp-containing solution.
2. Discussion of the Background
Trp is one of the essential amino acids. It is useful as a food additive, as an additive to animal feed, in medicine or for producing health foods.
In processes for producing Trp by fermentation using bacterial cells, glucose is employed as a main starting material. The crude crystals of Trp which are produced by this method however contain impurities such as proteins and colored materials.
It is very difficult to remove these impurities, from the Trp product irrespective of their quantity, since they have high hydrophobicity. Conventional recrystallization for example does not result in a good purification of Trp, since such impurities are incorporated in the surface of the Trp crystals or between the crystals. Therefore tryptophan obtained by such process does not fulfill the standard requirements for products which are to be employed, e.g., for medical supplies, which must have high purity (characterized by a transmission of 95% or more at .lambda.=430 nm).
Hence it has become necessary to develop a purification method by which highly purified non-colored Trp with a transmission of 95% or more is obtainable from a Trp-containing reaction solution.
In a known method (Japanese laid open patent application No. 895/1983) crude tryptophan is purified by treating an aqueous solution containing crude tryptophan with a non-polar, highly porous resin and by subsequent filtration with an ultrafiltration membrane to remove the reaction impurities which have accumulated in the solution. It has been found that impurities having a molecular weight of 500 to 5000 are responsible for the decrease of the transmission value. The method using an ultrafiltration membrane (i.e. according to Japanese laid open patent application No. 895/1983) cannot remove such impurities.
According to another known method (Japanese laid open patent application No. 39857/1984) the solution containing crude Trp is adjusted to an alkaline pH value and is crystallized by neutralization in the presence of a lower aliphatic alcohol or a ketone. In this process the degree of removal of impurities is insufficient and it is difficult to obtain crystals having a transmission value of 95% or more.
According to a further process (Japanese laid open patent application No. 126070/1986), a solution of Trp obtained by an enzymatic reaction, is heated to a temperature of 95.degree. to 100.degree. C. at an acidic pH in the presence of activated carbon. The solution is separated from the solid materialby filtration. The filtrate containing Trp is contacted with a non-polar highly porous resin and concentrated. After this operation, a lower aliphatic alcohol is added to the solution. In this method, however, decomposition and formation of colored material are caused when the reaction solution produced by fermentation and containing particularly large amounts of impurities is heated to a temperature of 95.degree. to 100.degree. C. in the presence of activated carbon. This is due to the instability of the indole ring of tryptophan.
There is therefore a need for a process for obtaining purified tryptophan from an aqueous solution containing this material, where the process provides a high quality product and does not suffer from the disadvantages discussed above.